1. Field of the Invention
The present invention relates to the pixel circuit for active matrix of current driving device. More particularly, one important issue of this invnetion is how to reduce the variation of the current which is sent to the current driving device.
2. Description of the Prior Art
The current driving devices, such as the organic light emitting device (OLED) and the polymer emitting device (PLED), are getting more important in contemporary electronic devices. In general, in order to reduce consumed current and to prolong the lifetime, the conventional technique usually uses the pixel circuit of the active array to provide the current required by the current driving device.
As shown in FIG. 1, the conventional pixel circuit of active matrix at least has following elements: first transistor 11, second transistor 12, third transistor 13, fourth transistor 14, capacitor 15, and current driving device 16.
As shown in FIG. 1, the source and the gate of first transistor 11 are separately electrically coupled with first terminal 101 and second terminal 102. The drain and the gate of second transistor 12 are separately electrically coupled with the drain of first transistor 11 and third terminal 103. The source and the drain of third transistor 13 are separately electrically couple with constant voltage source (constant voltage source) 17 and the drain of first transistor 11. The source and the gate of fourth transistor 14 are separately electrically coupled with constant voltage source 17 and the gate of third transistor 13. Two plates of capacitor 15 are separated electrically coupled with constant voltage source 17 and the gate of third transistor 13. Current driving device 16 is electrically coupled with the drain of fourth transistor 14. Besides, for a complete array, first terminal 101 usually is electrically coupled with the data line which delivers the current signal, but second terminal 102 and third terminal 103 usually are electrically coupled with the scan line which delivers the voltage signal.
Clearly, whenever both first transistor 11 and second transistor 12 are turned on, the current delivered from constant voltage source 17 is delivered through both third transistor 13 and first transistor 11 into the data line. Besides, second transistor 12 also is turned and the voltage/charger is storaged in both third transistor 13 and capacitor 15, and then a current also is delivered through fourth transistor 14. In the mean time, because the gates of both transistors 13/14 are electrically coupled with capacitor 17, especially with the plate where voltage is different from the voltage of constant voltage source, a current mirror is formed. Herein, the current quantity ratio between third transistor 13 and fourth transistor 14 is directly proportional to the width-length-ratio ratio of these transistors 13/14, and the stored voltage of capacitor 15 is equal to the voltage difference between the age and the source of third transistor 13 (or fourth transistor 14). Hence, by adjusting the width-length-ratio of these transistors 13/14, or by adjusting the voltage which applied to the gates of these transistors 13/14 from the drain of second transistor 12, the current delivered to current driving device 16 could be properly controlled.
In contrast, whenever both first transistor 11 and second transistor 12 are turned off, no current could be delivered from constant voltage source 17 through third transistor 12 to any terminal, and no current could be delivered through the drain of second transistor 12 to turn on both third transistor 13 and fourth transistor 14. However, owing to capacitor 15 could maintain the stored voltage and apply it to the gates of both transistors 13/14, fourth transistor 14 still is turned on. In particularly, owing to the voltage different between two plates of capacitor 15 is equivalent to the difference between the gate and the source of third transistor 13 (four transistor 14) while both first transistor 11 and second transistor 12 are turned on, the current quantity delivered into current driving device after both first transistor 11 and second transistor are turned off would be equivalent to the current quantity delivered into current driving device while both first transistor 11 and second transistor are turned on.
However, the quality of any real element almost is different to that of the ideal element. For example, the parasitic capacitance between the gate and the source (or drain) of the real transistor usually is not zero, especially while the real transistor, such as OLED, being formed in and on the low temperature polysilicon substrate, any charger appears on the source and/or the drain would affect the real gate voltage. For example, for a turned-on real transistor, not only some chargers are located inside the gate for the existence of the parasitic capacitance but also some charges are located in the channel under the gate. Thus, while the real transistor being turned off, these chargers would not be constructed by the gate voltage and be delivered to both the source and the drain of the real transistor, and then an extra current is induced. These problems, usually are called as the switch effect or the charger coupled effect, would let practical application of the pixel circuit shown in FIG. 1 have two following defects:
First defect, while both first transistor 11 and second transistor 12 being turned off, the current delivered through third transistor 13 would be terminated. Thus, some chargers would be accumulated in and raise the voltage of the drain of third transistor 13. In the mean time, if the capacitance between the drain and the gate of third transistor 13 is irnegligible, the irnegligible capacitor would couple the chargers in the drain of third transistor 13 with capacitor 15 (or the plate where voltage is different from the voltage of constant voltage source 17) and let the stored voltage of capacitor 15 be changed.
Second defect, while both first transistor 11 and second transistor 12 being turned off, all chargers inside and under the gates of these transistors 11/12 would be delivered into the sources and the drains of these transistors 11/ 12. In this way, some chargers are delivered to the drain of third transistor and induce the previous defects, some chargers are directly delivered through the drain of second transistor 12 to capacitor 15, especially to the plate which is not directly coupled with constant voltage source 17, and change the stored voltage of capacitor 15.
Significantly, even the capacitance of the gate of each transistor is zero, even the quality of each transistor is alike to that of the ideal transistor, the switch effect of second transistor 12 still would change the stored voltage of capacitor 15. Moreover, if the gate capacitance of each transistor is irnegligible, the stored voltage of capacitor 15 would be strongly changed by the switch effect, or the turned off process, of both first transistor 11 and second transistor 12.
Accordingly, because the output of current driving device is strongly dependent on the inputted current, and because the current delivered through fourth transistor 14 to current driving device 16 is strongly controlled by the stored voltage of capacitor 15 for the gate of fourth transistor being electrically coupled with capacitor 15, how to ensure the stored voltage of capacitor 15 is stable and exact, especially is independent on the states of other transistors 11/12/13, is a red-hot and unsolved topic.